Particularly among industrial robots, there are some which utilize hydraulics as drive means thereof. However, the robots generally employ servo motors and the torque of the servo motors is usually transmitted to a robot arm by way of a speed reducer.
Meanwhile, in the teaching playback robot, the plurality of points on a path along which the robot arm passes are preliminarily taught and, at the time of playback, the robot is controlled by an interpolation of teaching points so that the robot arm moves along the path from one teaching point to another teaching point.
Conventionally, when the robot moves from one teaching point to another teaching point, once the robot reaches a target point, the robot is stopped without considering the conditions of the loads applied to the respective shafts of the robot.
Therefore, it is often the case with any one of the robot shafts that when the movement of the arm from one teaching point to another teaching point is in a direction to lift a load, the robot shaft is stopped in the load lifting condition even after the completion of the movement.
FIG. 1 exemplifies such a movement, wherein numeral 1 indicates a first shaft, numeral 2 indicates a second shaft, numeral 3 indicates a third shaft and capital letter W indicates a load. In FIG. 1, when the respective shafts move from positions shown with a solid line to positions shown with a hyphenated line which represents subsequent teaching points, the second shaft 2 moves in a direction to lift the load W and is stopped in the load lifting condition. Here, no brake means is provided for maintaining the stopped condition.
If the drive system of the above shafts comprises a servo motor 4, a speed reducer 5 and an arm 6 comprised of any desired shafts, the transmission of the torque from the servo motor 4 to the arm 6 is expressed as follows. EQU T.sub.L =T.sub.MO .times..eta.i
.thrfore.T.sub.MO =T.sub.L /(.eta..times.i) (1)
wherein,
T.sub.MO : required motor torque PA0 .eta.: normal efficiency of the speed reducer (.eta.&lt;1) PA0 1/i: speed reduction ratio PA0 T.sub.L : torque on arms by loads PA0 T.sub.M1 : required motor torque PA0 .eta.': reverse efficiency of the speed reducer (.eta.'&lt;1)
Meanwhile, the torque transmission from the load W to the servo motor 4 is expressed as follows. EQU T.sub.M1 T.sub.L .eta.'.times.(1/i) (2)
wherein,
As described previously, since the normal efficiency .eta. (&lt;1) is the inverse of the reverse efficiency .eta.'(&lt;1), the load applied to the servo motor 4 for driving the shaft greatly differs between a case (a) in which the arm is moved in a load lifting direction and then is stopped while maintaining the load lifting condition and a case (b) in which the arm is moved while holding the load and then is stopped.
Therefore, in the case (a), the servo motor consumes a considerable amount of electricity and eventually increases the generation of heat which will adversely affect the accurate operation of the motor.
Accordingly, the task to be achieved by this invention lies in stopping the robot under the condition (b), thus reducing the required torque of the servo motor and the generation of heat.